i like the idea of digging 3 ft hole and inserting 4x4 , then 1 ft of dirt to keep it verticle on plum
& then 2 ft of dry cement mix & then water.
But , Im lazy & my yard is full of rocks under the top soil.
so, my answer is to only dig down 18 inches with a 6 inch or 7 inch dia. hole and wedge in some rocks to keep the 4x4 verticle, then either dry cement & then water or mixed cement.
one advantage of only a 18 inch deep hole is its deep enough to hold the pole and leaves you
with a 6 ft 4in upright to attach the roof of a 8x8 shed to, just in case your making a wood shed
out of stockade fencing for walls.
second advantage of an 18 in hole is it dont take so long to dig and your not so tired when its dug .
third advantage is you dont need so much cement.
Anyways, it works for me. I have not had any poles fall down in the 4 years since i installed the first one, despite considerable rain fall.
hey , keep digging 3 ft deep holes, its not my back thats hurting from 2 much digging and im not paying for the extra cement or buying 12 ft long 4x4 at double the price.
But your deep holes are twice as strong, 2 give credit where credit is due.
for me strong enough is good enough because im not biulding a bridge over the missippi river.